The production of metal droplets is useful in a variety of research and commercial applications. Such applications include metal powder production, rapid solidification research, spray forming of discrete parts, spray forming of strips, spray forming of metal-matrix composites and metal coating. In carrying-out these applications, there are a variety of methods used to produce the metal droplets such as atomization of molten metal by gas jets or by high pressure water, spraying molten metal onto a spinning disc (melt spinning) or into a vacuum to form discrete particles, vaporization of metal in a vacuum followed by condensation, fusion of metal in a vacuum followed by condensation, fusion of metal by an electric arc followed by the formation of droplets which are forced out of the arc zone, and forming a molten surface on a metal rod and agitating the metal at an ultrasonic frequency.
Another technique to generate metal droplets, particularly for research purposes, is electrohydrodynamic (EHD) spraying. The EHD technique comprises the use of a very intense electric field at the tip of a capillary tube through which molten metal flows. The electrostatic stresses applied by the electric field at the tip of the small capillary tube result in a highly dynamic process at the charged liquid surface, resulting in charged droplet formation. EHD processes and variations thereto are disclosed in U.S. Pat. No. 4,264,641 and "Application of Electrohydrodynamic to Rapid Solidification of Fine Atomized Droplets and Splats," Perel et al, Mar. 23-26, 1980, at the Conference on Rapid Solidification Processing, Principles and Technologies, II, Reston Va.
While each of these known processes have their advantages and have achieved varying degrees of success, none of them is capable of producing with any consistency metal droplets uniform in size, shape, initial velocity, and thermal state.
Ink jet printing processes, while producing uniform liquid droplets, are not concerned with producing charged uniformly sized metal droplets. Also, maintaining a separation between droplets is not a problem or an issue in ink jet printing because the distance from the ink nozzle to the printing surface (paper) is no more than a few centimeters. This is unlike metal droplet processes wherein the distance from droplet formation to the substrate or collector needs to be sufficiently extended for the metal droplets to cool and at least partially solidify. As such the distance generally must be at least about 25 centimeters. At such a distance, droplets in a stream broken from a jet would naturally merge with one another, with the merging destroying any uniformity of initial droplet distribution.
Accordingly, it is an object of the present invention to develop an apparatus and process for producing charged uniformly sized metal droplets. By virtue of the charge, droplets are prevented from merging in flight and thus they can remain uniformly sized until they solidify or are collected on a substrate. Furthermore, the charge on the droplets makes it possible to manipulate the flight of the droplets with externally applied electric fields.
It is another object of the present invention to produce charged uniformly sized metal droplets for use in research and commercial applications.